Chapter 1 - Basics of rainwater harvesting

Rain Water Harvesting is essentially the capture of rainwater where it falls. There are two main techniques of rainwater harvesting, namely:

Storage for future use

Recharge into the ground

Water can be collected either from rooftops or from the ground or a combination of both. Rainwater harvesting systems can vary widely in scope and complexity. It can be the simple collection of rainwater from the rooftop of a house for use in gardening, or collection from a large school campus for use in the toilets or recharge into the ground.

Figure 1.1: Storage of rainwater Figure 1.2: Storage and recharge

Benefits of rainwater harvesting

Rainwater harvesting has a number of benefits both at an individual level and at the city-wide level.

It would bring down consumer utility (water supply) bills and this is of great value especially to institutions, which spend considerable sums on water.

Rainwater recharged into the ground would have a positive impact on groundwater quality through dilution of fluorides, nitrates and salinity. It would also stop the decline in groundwater levels.

Rainwater has nearly neutral pH and has zero hardness. These properties make it very favourable for use in a variety of applications in homes, institutions and commercial establishments and industry. If stored for future use, rainwater can function as a useful supplementary supply, thus reducing the stress on public water supply sources

Figure 1.3: Storage and recharge

In coastal areas, where there is excessive groundwater extraction, recharge of rainwater into the ground would prevent sea-water ingress into fresh water aquifers.

Figure 1.4: Recharge of Rainwater

Urban flooding can be controlled if the residents of a city harvest rainwater from their rooftops for future use or take steps to effectively recharge groundwater within their premises.

Using harvested rainwater reduces water demand from the municipality, which in turn reduces energy consumption in the water distribution network.

You must know…..

Water in lakes, ponds, rivers and oceans is called surface water.

Almost 30% of the world’s fresh water reserves are found under the ground. This water, called Ground water, is present in spaces between soil and gravel particles and cracks in geological rock formations. It is not in the form of underground streams. In some places groundwater can dissolve limestone to form caverns and large openings but this is unusual.

When it rains, some of the water soaks into the soil passing between soil, gravel and rock until it reaches a depth below which the Earth is saturated with water. This depth is called the water table. Above it is the unsaturated zone and below it the saturated zone. Groundwater can be found almost everywhere but the depth at which it can be found (the water table) differs from place to place. The water table is dependent on many factors. Heavy rains can cause it to rise and considerable extraction of groundwater, as is happening in most urban areas, may cause it to fall.

Some parts of the saturated zone contain more water than others. These parts are usually made up of very permeable material and are called aquifers. Aquifers yield useful quantities of water when tapped by a well. Aquifers can be of two types – unconfined aquifers and confined aquifers. The upper boundary of an unconfined aquifer is the water table. If water in an aquifer is confined between rock layers of relatively low permeability, the aquifer is termed as confined.

Recharge or infiltration is process of surface water seeping into the ground (from surface water systems and rain). The rate of recharge depends on the geology of the place. The rate of recharge would be very low through relatively impermeable materials like clay and shale and faster through more porous sandy soils.

Figure 1.5: SalineWater Intrusion

When it rains, only a portion of the water seeps into the ground. Most of it flows downhill over the land surface and is termed as runoff. Runoff is what keeps rivers and lakes full of water. Rainwater that flows off rooftops is called rooftop runoff.